1. Field of the Invention
The present invention relates to communications control methods and, more specifically, to a method for controlling communications among a plurality of terminals coupled to each other to form a network for serial transmission of a mixture of isochronous data and anisochronous data.
2. Description of the Background Art
In recent years, computers, peripheral devices, digital video devices, and other components are often connected to each other in homes and offices to form a local area network. On the network of this type, a mixture of isochronous data recurring at periodic time intervals (for example, video and audio data for streaming reproduction) and anisochronous data (for example, burst-like communications data) is transmitted in serial.
One example of the known standards for serial transmission of such a mixture is IEEE 1394. In IEEE 1394, all components forming a network are under communications control as described below.
FIG. 23 is a diagram showing an example of the structure of a conventional local-area network 209 that complies with the IEEE 1394 standard. FIG. 24 is a schematic diagram showing a conventional communications control method according to the IEEE 1394 standard. An example of the structure of a communications control cycle according to the IEEE 1394 standard is shown in (A) of FIG. 24, and an example of communications control carried out on the network of FIG. 23 is shown in (B) of FIG. 24.
In FIG. 23, the conventional network 209 includes a controller 210, a digital video player 211, a digital television 212, a set-top box (hereinafter, STB) 213, and a computer 214. These components 210 to 214 are connected in serial (or in tree shape) to each other via a cable that complies with IEEE 1394. In the network 209, the controller 210 controls the other components 211 to 214.
In the above structured network 209, assume herein that isochronous data “I1” is going to be transmitted from the digital video player 211 to the digital television 212; isochronous data “I2” is going to be transmitted from the STB 213 to the computer 214; and anisochronous data “N” is going to be transmitted from the computer 214 to the digital video player 211.
In IEEE 1394, as shown in (A) of FIG. 24, a control time is divided into predetermined cycles (for example, every 125 μs), and each cycle has a predetermined isochronous region of a predetermined time length (for example, 100 μs at maximum). The isochronous region is further divided into plural (two, in this example) regions. These two regions are respectively assigned to the components having isochronous data to be transmitted. In the example, one divided region is assigned, as a dedicated region (channel 1), to the digital television 212, while the other divided region is assigned, as a dedicated region (channel 2), to the STB 213.
As shown in (B) of FIG. 24, before the start of transmission, the controller 210 reports, to each component, information 200 about the dedicated regions assigned to these components. The information 200 includes times when each isochronous data is to be transmitted. When transmission starts and enters into the cycle, the controller 210 first transmits a packet 201 indicating the start of the cycle to each component. Upon receiving the cycle start packet 201, the components having the isochronous data to be transmitted (here, the digital video player 211 and the STB 213) transmit the isochronous data (I1 and I2, respectively) by using the dedicated region of their own (channel 1 and channel 2, respectively).
When the procedure exits the isochronous region, the controller 210 provides a transmission instruction 202 to the component having the anisochronous data (here, the computer 214). Upon receiving the instruction from the controller 210, the computer 214 transmits the anisochronous data (N).
Next, the destination of the anisochronous data (N) (here, the digital video player 211) receives the anisochronous data, and then returns a response packet 203 indicating whether or not the data has been successfully received, to the data originating terminal (the computer 214) and the controller 210. Upon receiving the response packet 203 from the computer 214, the controller 210 determines whether or not retransmission is required. In this example, the transmitted response packet 203 indicates that the data has been successfully received. Therefore, the controller 210 determines that retransmission is not required. Then, when the procedure exits the cycle and then enters into another, the controller transmits a packet indicating the start of the next cycle to each terminal (such packet transmission procedure is not shown in FIG. 24), and then the procedure is repeated similarly thereafter.
On the other hand, if the response packet 203 returned from the digital video player 211 indicates a reception error, the controller 210 transmits, to the computer 214, an instruction for retransmitting the anisochronous data (N). Then, the digital video player 211 returns response packets to the computer 214 and the controller 210. If the response packet from the digital video player 211 indicates a reception error, the controller 210 again instructs the computer 214 to retransmit the data (such retransmission procedure is not shown in FIG. 24).
As such, according to the IEEE 1394 standard, the control time is divided into cycles, and each cycle has an isochronous region of a predetermined time length allocated. This isochronous region is further divided into dedicated regions to be assigned to the components having isochronous data. Therefore, each of these components can transmit the isochronous data one time per cycle. Consequently, the isochronous characteristics can be kept.
On the other hand, the region other than the isochronous region in each cycle is an anisochronous region, where the components having anisochronous data to be transmitted are controlled (asynchronous control) so that they carry out transmission one after the other. Thus, serial transmission of mixed isochronous and anisochronous data can be achieved.
In IEEE 1394, even if a reception error of the isochronous data occurs, retransmission control is not carried out. Instead, the ratio of error occurrence is reduced to less than a predetermined value by restricting the length of a cable connecting the components together to less than a predetermined length (4.5 m if a conductor cable). With the ratio of error occurrence less than the predetermined value, quality deterioration in image and sound can be suppressed to such an extent that the user cannot recognize the deterioration.
In recent years, a wireless connection between components has become more desired. A wireless connection dispenses with the time and trouble of wiring, and enables the user to use each component wherever he/she desires.
In a wireless transmission path, however, transmission errors are prone to occur more, as compared with a wired transmission path. The ratio of transmission error occurrence is significantly increased if the components are spaced far apart or an obstacle is located therebetween. As a result, it is highly possible that images may be disturbed during streaming reproduction and noise may be mixed in sound.